It is known that LED lighting devices normally use switching supplies which allow, among other functions, to regulate the output intensity according to the user's commands.
A regulation mode of proven efficacy contemplates the use of a double pulse width modulation (or PWM) control.
Switching supplies are based on a first high-frequency PWM control by means of which the current which flows through the LED lighting elements is maintained about a reference value. More in detail, in LED lighting apparatuses, the switching supply comprises a switch, normally a MOSFET, connected between an input supply line and the LED lighting elements, and a control circuit. An inductor, connected between the switch and the LED lighting elements, is charged when the switch is closed and is discharged through the LED lighting elements and a recirculation diode when the switch is open. The control circuit, high-frequency control signal PWM (generally higher than 1 MHz), alternatively opens and closes the switch according to a duty-cycle determined according to the current absorbed by the LED lighting elements and to a reference, so as to control the charging and discharging of the inductor. The current which flows through the lighting elements is thus maintained about a desired operative value.
In order to vary the lighting intensity, a second low-frequency PWM control is used (e.g. from 100 Hz to 1 kHz). A second PWM signal, e.g. supplied by an external control unit, alternatively enables and disables the switching of the switch according to a duty-cycle fixed by the user through a command. In practice, during a portion of each period (active phase or “on” phase), the switch is controlled as described above and switches at high frequency. During the remaining portion of the period (inactive phase or “off” phase) the switch is deactivated and remains open, regardless of the conditions of the LED lighting elements. Once the inductor is completely discharged, the passage of current crossing the LED lighting elements ceases and the LEDs are cut off. The average current crossing the LED lighting elements and thus the lighting intensity are determined by the duty-cycle of the second PWM signal and by the current operating value when the switch is enabled.
Although very simple and effective, the use of the double PWM control for regulating the output intensity of LED lighting devices has some serious limitations.
As mentioned, in particular, the LEDs are cut off when the switch is deactivated by the low-frequency PWM signal. The lighting of the LEDs during the subsequent cycle causes a current peak which is short lasting but has considerable width, and is in all cases much higher than the usual operating current of the active phases, in which the switch is enabled. The lighting peaks subject the LEDs to stress which, given the very high number of cycles, may be damaged over time. On the other hand, the frequency of the second PWM signal cannot be reduced beyond a given limit because this would produce a flickering perceivable by the human eye. Therefore, a consequence of the type of described control is the reduction of the life of the LED lighting elements.